Global, direct and diffuse measurements of erythemal UV radiation with UV-Biometer and radiative transfer calculations are used to estimate the influences of (1) a snow-covered surface, (2) an overcast sky, and (3) a joined snow-covered surface and overcast sky at Davos (1610 m asl, Swiss Alps). The influence of total ozone, zenith angle, and aerosol are first investigated. The influence of surface albedo and clouds are then estimated as a function of the zenith angle after normalization of the data to a fixed ozone amount. If the ground is covered with snow on clear-sky days at Davos, erythemal irradiance increases by 15 to 25% due to multiple reflections between the surface and the atmosphere. This relative increase may reach 80% on overcast days. The highly significant dependence of the albedo on solar elevation is most probably due to inhomogeneities in the surrounding terrain leading to a strong non-Lambertian behavior. The impact of clouds on radiation is highly variable: on overcast days with a snow free surface, erythemal UV radiation at Davos is reduced to a level ranging between 8% (very thick cloud cover) and 70% (thin cloud layer) relative to the radiation under clear-sky conditions. ¿ 2000 American Geophysical Union